Solution processing: fabrication and characterization of polymeric nanocomposite films and polystyrene nanoparticles

Official URL: http://risc01.sabanciuniv.edu/record=b1121594 (Table of Contents)

An approach to bottom-up design of new materials was developed starting from homogenous solutions. Solution processing techniques were used to fabricate advanced solid state materials, and processing parameters were identified and characterized. Three studies related to this work are reported herein: i) Polymeric electrospun nanofibers were metallized with transition metals for potential use as catalysts in organic reactions or sensing elements. Two different polymer-metal systems, which were palladium/poly(acrylonitrile-co-acrylicacid) and silver/poly(acrylonitrile-coglycidylmethacrylate), were employed. Polyacrylonitrile based copolymers were chosen as carrier material in view of their facile spinnability and established utility as precursor materials of carbon fibers. Nanofibers, in both cases, were obtained by electrospinning of homogeneous solutions in dimethylformamide. Metals were deposited on electrospun films starting from the metal salts by following two different procedures. In one route, palladium-II-chloride and the polymer were dissolved in dimethylformamide and subjected to electrospinning. Salt molecules were homogeneously distributed into nanofibers. Palladium cations were reduced after the electrospun film was immersed into an aqueous solution of hydrazine. The parameters affecting and tuning the particle size were determined. In particular, the amount of acrylic acid on the polymer backbone and palladium salt concentration in solution described two key factors. Palladium particles, called clusters, were afforded as polycrystalline structure consisting of smaller crystal units. Catalytic activity of palladium produced on electrospun film was investigated in a hydrogenation reaction of unsaturated alcohols. It was found that electrospun-supported palladium particles displayed 4.5 times higher catalytic activity than alumina-supported palladium. In the second route, silver was coated on poly(acrylonitrile-co-glycidylmethacrylate) nanofibers by use of electroless plating techniques. Reagent-accessible oxirane groups supported on the nanofibers were modified with a reducing agent, hydrazine. Surface-modified electrospun nanofibers were allowed to react with an ammonic solution of silver nitrate. A redox reaction took place during which time metallic silver was nucleated along the fiber surface, affording silver nanoparticles of 40 nm diameter. These particles featured typical separation distances of 5-50 nm. ii) Thermoelasticity of silica reinforced poly(dimethylsiloxane) networks was examined. Poly(dimethylsiloxane) networks exhibit rubber-like elasticity; that is, they recover their original state following deformation. Elasticity is an entropy driven phenomena for polymers. Uniaxial stretching of a network elongates the chains, resulting in a decreased conformational entropy due to the restricted number of low energy conformations that the extended chains can adopt. When the stress is removed, the chains recoil into the relaxed state with higher entropy. Elastomeric force, f, applied in uniaxial extension has an entropic and an energetic component affecting the network chains at the molecular level. The entropic component, fs, is used in changing the configurations of the chains into less disordered state. The rest of the force, fe, is used in changing the conformations of the chains. The ratio of fe/f can be determined by thermoelasticity experiments which are based on stress-strain measurements at constant volume. An ideal network was prepared from hydroxylended poly(dimethysiloxane) chains. They were dissolved in toluene. Fumed silica was introduced into the polymer solution prior to end-linking. A tetrafunctional crosslinker, tetraethoxysilane, was added into the homogenous solution and end-linked in the presence of Tin(II) 2-ethylhexanoate as a catalyst. The thickness of the nanocomposite film is on the order of 2 mm. The filler content was varied in the range 0-5 wt%. Tapping mode Atomic Force Microscopy was performed to characterize the silica particles, which become larger as the silica concentration increases. The temperature coefficient and the energetic part of the force in uniaxial extension are found to increase with increasing silica content. The elastic modulus of the reinforced networks was determined by mechanical experiments and swelling measurements. The modulus increases linearly with increasing silica concentration. iii) Amorphous polystyrene molecules/clusters were isolated and investigated. Erman and Flory showed that long polystyrene molecules undergo large dimensional changes in cyclohexane at 35°C. This event is known as coil-globule transition. Here the dimensional changes at dry state after the transition takes place were imaged and measured. Dilute solution of cyclohexane was cast on mica by the drop deposition technique. Solvent evaporation left behind a discontinuous film consisting of separated polystyrene islands. Atomic Force Microscopy was employed to determine the morphology and dimensions (volume, height and diameter) of the polystyrene particles. The experiment was performed at four different temperatures. It is found that the dimensions are strongly temperature dependent and exhibit a Gaussian-like distribution. Polystyrene chains tend to form clusters as the temperature increases. Two scenarios were discussed for whether the particles contain single or several chains.